The best way to take a baby’s temperature depends on age. For newborns through 3 months, a rectal reading with a standard digital thermometer is the most accurate method. As your baby gets older, you gain more options, but the technique matters just as much as the tool. Here’s what to know for each method and age range.
Which Method to Use at Each Age
For babies from birth to 3 months, a rectal temperature is the gold standard. Newborns can spike dangerous fevers quickly, and rectal readings give the most reliable core body temperature. Temporal artery thermometers (the ones you swipe across the forehead) may also provide accurate readings in newborns, but rectal remains the primary recommendation.
From 3 months to 4 years, you have three good options: rectal, armpit (axillary), or temporal artery. Rectal is still the most accurate, but an armpit reading works well as a quick screening. If the armpit reading seems high, you can follow up with a rectal check to confirm.
Digital ear thermometers are not reliable for babies under 6 months. A baby’s ear canals are too small and narrow for the sensor to get an accurate reading. Once your baby reaches 6 months, an ear thermometer becomes a reasonable option.
How to Take a Rectal Temperature
Clean the thermometer tip with 70% isopropyl alcohol before use, then rinse it under cool water to remove any alcohol residue. Apply a small amount of petroleum jelly to the tip for lubrication.
Lay your baby face down on your lap or on a firm, flat surface. You can also lay them face up and hold their legs toward their chest, similar to a diaper change position. Either way, keep one hand on the baby at all times so they can’t roll or squirm off. Gently insert the thermometer tip about half an inch to one inch into the rectum. Don’t force it. Hold the thermometer in place until it beeps, which usually takes about 10 to 20 seconds with a digital model.
A normal rectal temperature is around 98.6°F (37°C), though healthy readings can range from about 97°F to 100.3°F depending on the time of day and your baby’s activity level. A rectal temperature of 100.4°F (38°C) or higher is considered a fever.
How to Take an Armpit Temperature
Place the thermometer tip in the center of your baby’s armpit, making sure it touches skin and not clothing. Fold your baby’s arm down snugly against their body and hold it there until the thermometer beeps. This usually takes 15 to 30 seconds with a digital thermometer, though some models need a bit longer.
Armpit readings run about 1°F (0.5°C) lower than rectal readings because the sensor isn’t measuring core body temperature. An armpit temperature of 99°F (37.2°C) or higher generally suggests a fever, but the exact threshold matters less than the context. If your baby feels warm and an armpit reading is borderline, taking a rectal temperature gives you a more definitive answer.
Forehead and Ear Thermometers
Temporal artery (forehead) thermometers work by scanning the blood vessel that runs across the forehead. You swipe the sensor from the center of the forehead toward the hairline, following the manufacturer’s instructions for your specific model. These are fast and non-invasive, which makes them appealing for fussy babies. They can be used from birth onward.
Ear (tympanic) thermometers measure infrared heat from the eardrum. To get an accurate reading, you need to pull the ear slightly back and up to straighten the ear canal, then aim the sensor toward the eardrum. If your baby has been wearing a hat or headband covering their ears, remove it and wait a few minutes for the ear canal to return to its natural temperature before measuring. Ear thermometers are appropriate from 6 months of age onward.
Factors That Throw Off a Reading
Several things can give you a falsely high or low number. If your baby was just bundled in heavy clothing or blankets, their skin temperature may read higher than their actual core temperature. A recent warm bath can do the same. On the other hand, if your baby was just undressed for a while or exposed to a cool room, the reading may come in lower than expected.
The fix is simple: wait. Let your baby sit at normal room temperature for 10 to 15 minutes before taking a reading. Avoid checking right after a bath, right after a feeding, or right after they’ve been tightly swaddled. If you recently gave your baby a fever-reducing medication, keep in mind that the reading will reflect the medication’s effect, not necessarily whether the underlying illness has improved.
Cleaning Your Thermometer
Clean the thermometer both before and after every use. For digital thermometers, wipe the probe and body with a cotton ball or cloth soaked in 70% isopropyl alcohol. Don’t submerge the thermometer in liquid unless the packaging specifically says it’s waterproof. After wiping with alcohol, rinse the tip under cool running water to remove residue, then let it air dry or pat it dry with a clean cloth before storing it in its case.
If you use a non-contact forehead thermometer, wipe the sensor with an alcohol wipe and avoid harsh cleaning products that could damage the sensor. Keep a dedicated thermometer for rectal use only, and label it so no one accidentally uses it orally.
What Counts as a Fever
The fever threshold depends on how you measured. A rectal temperature of 100.4°F (38°C) or above is a fever. Armpit readings of 99°F (37.2°C) or above suggest a fever, though they’re less precise. Ear and forehead readings generally use the same 100.4°F threshold as rectal, but check your thermometer’s instructions since calibration varies by brand.
Age matters enormously when it comes to fevers. Any fever of 100.4°F or higher in a baby under 3 months old warrants an immediate call to your pediatrician, even if your baby seems fine otherwise. The immune system at that age is still immature, and infections can escalate quickly. For babies 3 months and older, the urgency depends more on how your baby is acting: whether they’re feeding normally, staying hydrated, and alert when awake.
For children under 2 years, a fever lasting more than 24 hours deserves a call to your doctor even if there are no other worrying symptoms. For children 2 and older, that window extends to 72 hours.
Tips for Getting an Accurate Reading
Take your baby’s temperature when they’re relatively calm. A screaming, squirming baby makes it harder to hold the thermometer in place and can slightly elevate body temperature from the exertion. If your baby is upset, try again in a few minutes.
Stick with the same method and the same thermometer when tracking a fever over time. Switching between rectal and armpit readings introduces variability that makes it harder to tell whether the fever is rising or falling. If you call your pediatrician, let them know which method you used so they can interpret the number correctly.
Wash your hands before and after handling the thermometer, especially when taking rectal temperatures. It’s a small step that reduces the risk of spreading germs between you and your baby.